Disc drives are data storage devices that store digital data in magnetic form on a rotating storage medium on a disc. Modem disc drives comprise one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Information is stored on the discs in a plurality of concentric circular tracks typically by an array of transducers (“heads”) mounted to a radial actuator for movement of the heads relative to the discs. Each of the concentric tracks is generally divided into a plurality of separately addressable data sectors. The read/write transducer, e.g. a magnetoresistive read/write head, is used to transfer data between a desired track and an external environment. During a write operation, data is written onto the disc track and during a read operation the head senses the data previously written on the disc track and transfers the information to the external environment. Critical to both of these operations is the accurate locating of the head over the center of the desired track.
A problem in disc drives that limits drive performance in general and head position accuracy specifically is component vibration or resonance. Components in the voice coil motor (VCM) plant of the disc drive exhibit resonance modes that adversely affect the performance of disc drive components. For example, because of resonance in the actuator arm, the transducer heads may not be positioned directly over the desired tracks indicated by the servo control of the disc drive. This problem is exacerbated by the recent push to increase the tracks-per-inch (TPI) on the disc surfaces. When TPI is increased, the room for margin in head placement becomes disproportionately smaller, and servo positioning errors become more frequent.
Unfortunately, resonance modes in the VCM structure cannot be completely eliminated without extreme cost. The presence of resonance modes in the VCM structure usually causes stability problems in the servo control loop. To overcome these problems, the servo controller is typically augmented with one or more notch filters. The notch filters attenuate the VCM structure response at the natural frequencies of the resonance modes. This combination of the servo controller with the notch filters preserves servo control loop stability but at the expense of closed loop performance.
In a traditional design, a set of fixed notch filters are designed and implemented for a large population of disc drives in a given drive family. In high TPI disc drives, the resonance modes tend to vary both from disc drive to disc drive within the population, and from head to head within each disc drive. Therefore, the fixed notch filters cannot guarantee that the resonance modes will be attenuated for all heads and all drives in the population for which they were designed. This reduces the effectiveness of the notch filters, and results in an increased rejection rate of disc drives.
Ideally, the notch filters will be specifically designed for each head of each individual disc drive. Recently, a method of notch filtering on a per-head basis was disclosed in U.S. Pat. No. 6,246,536, entitled “Notch Filtering as Used in a Disc Drive Servo,” issued to Paul Galloway, which is hereby incorporated for all that it teaches and discloses. Unfortunately, even with the Galloway solution, the inherent problems still persist with the use of notch filters. Essentially, notch filters cannot guarantee that the performance criteria will be met in the servo control loop. Two important performance criteria are servo bandwidth and servo runout, or positioning error. When more notch filters are added, for example, phase margin is reduced, which amplifies disturbances in the servo control loop. The amplified disturbances make the servo control loop prone to runout. Thus, while the use of notch filters with a servo controller can provide more stability in the servo control loop, they do so at the cost of performance.
Accordingly, there is a need for a method and apparatus for regulating the response of the VCM plant in a disc drive servo control loop to reduce the effects of unwanted resonance modes, guarantee loop stability, and minimize degradation of servo loop performance, without relying on the mechanism of notch filters.